A group-III nitride semiconductor is attracting attention as a semiconductor material for a next-generation high-frequency/high-power device, because the nitride semiconductor has a high breakdown electric field and a high saturation electron velocity. For example, an HEMT (high electron mobility transistor) device in which a barrier layer made of AlGaN and a channel layer made of GaN are laminated takes advantage of the feature that causes a high-concentration two-dimensional electron gas (2DEG) to occur in a lamination interface (hetero interface) due to the large polarization effect (a spontaneous polarization effect and a piezo polarization effect) specific to a nitride material (for example, see Non-Patent Document 1).
In some cases, a single crystal (a different kind single crystal) having a composition different from that of a group-III nitride, such as silicon and SiC, is used as a base substrate of an HEMT-device substrate. In this case, a buffer layer such as a strained-superlattice layer or a low-temperature growth buffer layer is generally formed as an initially-grown layer on the base substrate. Accordingly, a configuration in which a barrier layer, a channel layer, and a buffer layer are epitaxially formed on a base substrate is the most basic configuration of the HEMT-device substrate including a base substrate made of a different kind single crystal. Additionally, a spacer layer having a thickness of about 1 nm may be sometimes provided between the barrier layer and the channel layer, for the purpose of facilitating a spatial confinement of the two-dimensional electron gas. The spacer layer is made of, for example, AlN. Moreover, a cap layer made of, for example, an n-type GaN layer or a superlattice layer may be sometimes formed on the barrier layer, for the purpose of controlling the energy level at the most superficial surface of the HEMT-device substrate and improving contact characteristics of contact with an electrode.
In order to put into practical use the HEMT device or the HEMT-device substrate that is a multilayer structure used for preparation of the HEMT device, various problems have to be solved including problems concerning improvement of the performance such as increasing the power density and efficiency, problems concerning improvement of the functionality such as a normally-off operation, fundamental problems concerning a high reliability and a price reduction, and the like. Active efforts are made on each of the problems.
For example, it is known that, in a case where a nitride HEMT device has the most general configuration in which a channel layer is made of GaN and a barrier layer is made of AlGaN, the concentration of a two-dimensional electron gas existing in an HEMT-device substrate increases as the AlN mole fraction in AlGaN of the barrier layer increases (for example, see Non-Patent Document 2). If the concentration of the two-dimensional electron gas can be considerably increased, the controllable current density of the HEMT device, that is, the power density that can be handled, would be considerably improved.
Also attracting attention is an HEMT device having a structure with reduced strain, such as an HEMT device in which a channel layer is made of GaN and a barrier layer is made of InAlN, in which the dependence on a piezo polarization effect is small and almost only a spontaneous polarization is used to generate a two-dimensional electron gas with a high concentration (for example, see Non-Patent Document 3).
As for the normally-off operation, from the viewpoint of fail-safe, it is generally desirable that an electronic device, and particularly a power semiconductor device that handles a power control, performs a normally-off operation, that is, an operation that blocks conduction when no electrical signal is not inputted from the outside. On the other hand, an HEMT device made of a group-III nitride semiconductor is a device that uses a two-dimensional electron gas generated at a hetero interface as described above. Therefore, in an normally-on operation rather than the normally-off operation, the HEMT device originally exerts excellent conduction characteristics, that is, a low on-resistance. As a method for achieving the normally-off operation of the HEMT device made of a group-III nitride semiconductor, the following methods are known.
As for a nitride HEMT device of Schottky gate structure type including a channel layer made of GaN and a barrier layer made of AlGaN, for example, there are known: (1) a method in which the thickness of AlGaN barrier layer is reduced so that a gate threshold voltage (hereinafter, also referred to simply as a threshold voltage) is shifted in a positive direction, and thereby the normally-off is achieved (for example, see Non-Patent Document 4); and (2) a method in which recess etching is performed only in a portion immediately below a gate electrode (for example, see Non-Patent Document 5).
Alternatively, there are also known (3) a method in which, instead of a Schottky junction, a MIS (metal-insulator-semiconductor) structure with interposition of an insulating layer is adopted in an HEMT device of recess gate structure type (for example, see Non-Patent Document 6 and Non-Patent Document 7); and (4) a method in which an HEMT device having an inverted channel structure is prepared using an MIS gate structure (for example, see Non-Patent Document 8).
Moreover, there is also known (5) a method in which a channel layer is made of AlGaN whose Al mole fraction in all the group-III elements is 0.3 while a barrier layer is made of InAlGaN whose composition is in a predetermined composition range, to thereby achieve an HEMT device having a two-dimensional electron gas concentration of 2×1013/cm2 or higher and capable of the normally-off operation (for example, see Patent Document 1).
The above-described methods for achieving the normally-off operation in the HEMT device, except the method (5), involves problems that a manufacturing process is troublesome and that a sufficiently-low on-resistance is not obtained.
For example, in a case of the method (1), the reduction in the thickness of the barrier layer lowers the two-dimensional electron gas concentration. As a result, a low on-resistance, which is the original feature of the nitride HEMT device, cannot be obtained. The reason therefor is considered as follows. As the thickness of the barrier layer decreases, the distance between a channel portion and a surface of the barrier layer decreases. As a result, the potential of a surface level contributes to generation of electric charges, or a piezo polarization effect is reduced.
In the method (2), the adding of recess processing makes the process troublesome. To ensure the reproducibility in a device manufacturing process (to enable a device having a certain quality to be stably manufactured), a high accuracy of the recess processing is demanded.
The methods (1) and (2) are directed to an HEMT device of Schottky gate structure type, in which an upper limit of a positive voltage that can be applied to the gate electrode is determined by the height of a Schottky barrier. When a gate positive voltage is set to be about 1.5V or more, it is difficult to ensure a large drain current while suppressing a gate leakage current. On the other hand, the HEMT device has a feature that, when the HEMT device is designed to have a wide range of gate voltage application, a drain current thereof is increased. For example, in a case where the threshold voltage is −3V, the gate voltage range spans 4.5V, that is, from −3V to about +1.5V. However, in a case of the HEMT device designed such that the threshold voltage thereof is a positive value (>0V) by reducing the thickness of its barrier layer, the gate voltage range spans, at most, about 1.5V. In this case, while a maximum drain current (on-current) of the former is about 0.8 A/mm, that of the latter is about 0.4 A/mm or less. Such a reduction in an on-current is more noticeable as the shift of the threshold voltage to the positive side is larger. Accordingly, when the normally-off operation is performed in a nitride HEMT device having a Schottky gate, a problem arises that a wide gate voltage range is not ensured and therefore a large drain current does not flow (the on-resistance cannot be lowered), resulting in a failure to obtain good conduction characteristics.
In the method (3), the recess processing and the insulating film formation process are added, which makes the process troublesome. To ensure the reproducibility in a device manufacturing process (to enable a device having a certain quality to be stably manufactured), a high accuracy of the recess processing is demanded.
The method (4) requires the step of forming the MIS gate structure. Moreover, the electron mobility in the inverted MIS channel structure is low, namely, 200 cm2/Vs or less. Therefore, even when the normally-off operation is achieved, the performance of the HEMT device itself is degraded.